(1) Field
This embodiment relates to a method for fabricating semiconductor devices and, more particularly, to a method for efficiently fabricating semiconductor devices.
(2) Description of the Related Art
In recent years ferroelectric memories, such as ferroelectric random access memories (FeRAMs), in which a polarization inversion of a ferroelectric material is used for holding information in a ferroelectric capacitor have been developed. An FeRAM is a nonvolatile memory. That is to say, information held is not lost when power is turned off. FeRAMs are expected to realize high integration levels, high-speed operation, great durability, and low power consumption.
A thin film, such as a Pb(Zr,Ti)O3 (PZT) film or an SrBi2Ta2O9 (SBT) film, which has a perovskite crystal structure and in which there is a large amount of residual dielectric polarization is chiefly used as a material for a ferroelectric layer included in a ferroelectric capacitor.
However, it is known that the characteristics of such a ferroelectric layer deteriorate due to water which enters from the outside through an interlayer dielectric. A material, such as a silicon oxide film, which has a high affinity for water is used for forming the interlayer dielectric. Water which enters the interlayer dielectric decomposes into hydrogen and oxygen in a step in which an interlayer dielectric or a metal wiring is formed at a high temperature. Hydrogen reacts with oxygen contained in the ferroelectric layer and oxygen defects are formed in the ferroelectric layer. As a result, the crystallinity of the ferroelectric layer decreases. This is the reason why the characteristics of the ferroelectric layer deteriorate. It is known that using a ferroelectric memory for a long period also causes a deterioration in the crystallinity of the ferroelectric layer. As a result, the amount of residual dielectric polarization in the ferroelectric layer or the dielectric constant of the ferroelectric layer decreases and the performance of the element falls off.
To prevent the deterioration in the performance of the element, an alumina film for preventing water or hydrogen from entering a ferroelectric layer at the time of fabricating a ferroelectric memory is formed in a semiconductor device. For example, in order to prevent water or hydrogen from entering a ferroelectric layer, an alumina film is formed so as to envelop a ferroelectric capacitor (see, for example, Japanese Patent Laid-Open Publication No. 2006-108268). In addition, in order to prevent water or hydrogen from entering a semiconductor element from the top, an alumina film may be formed over a first wiring.
As stated above, a deterioration in the characteristics of the ferroelectric layer is caused by water or hydrogen. In addition, it is known that a deterioration in the characteristics of the ferroelectric layer is caused by plasma chemical vapor deposition (CVD) performed at the time of forming an oxide film, plasma ashing performed at a high temperature for a long time, plasma nitriding, and the formation of a tungsten plug. In this case, the main reason for the deterioration in the characteristics of the ferroelectric layer is that a ferroelectric capacitor is left in an atmosphere of high density plasma.
In order to prevent the deterioration in the characteristics of the ferroelectric layer, a thin film formation method in which the density of plasma is decreased, a thin film formation method in which time for which a ferroelectric capacitor is exposed to an atmosphere of plasma is shortened, or a thin film formation method in which a tungsten plug is formed at a temperature lower than the usual temperature is used. In addition, recovery annealing treatment is performed in order to prevent the deterioration in the characteristics of the ferroelectric layer (see, for example, Japanese Patent Laid-Open Publication No. 2006-100405).
However, even if the measures disclosed above are taken after the formation of the ferroelectric layer, there is variation in the characteristics of ferroelectric capacitors among wafer substrates in the same lot and there is variation in the retention characteristics (data holding functions) of elements among the wafer substrates. The variation in the retention characteristics of the elements among the wafer substrates leads to a decrease in the efficiency of manufacturing semiconductor devices each having a ferroelectric capacitor.